The present invention refers to a fiber composite plastic preform for the manufacture of curved profiles with any fiber layer structure. The invention further refers to a method for producing curved profiles from fiber composite plastic with any fiber layer structure.
The production of curved profiles from fiber composite plastic (FVK) with a profile-directed fiber coordinate system, such as a system including a 90° ordinate to the profile curve and a 0° abscissa tangential to the curved profile, represents a great production challenge. Due to increased requirements regarding the fiber content and the demand for high quality parts in the field of aircraft primary structures, the use of FVK-preforms that are provided in either so-called “prepregs” or as non-impregnated semi-finished fiber products, are considered standard. These fiber layered structures can be brought into the desired shape via a hot forming process. Prepregs are semi-finished textiles pre-impregnated with reaction resin, for manufacture of construction parts that are hardened under heat and pressure.
Upon forming the fiber layers, due to the changes of the radius of the molded arcuate segments, “stretching” or “compression” of the material inevitably happens, which requires a length compensation within the single fibers layers. In layers with a fiber angle of φ≠0°, this is carried out through a selective shift of the single fibers. Changes in the arch length are then balanced through “spreading apart” or “pushing together” of a portion perpendicular to the fiber direction. In the case of a pushing together, a correspondingly stretched deposit of angled layers must be taken into account. For fiber layers with φ=0°, this solution is not applicable, although these profile-directed fiber layers are oftentimes of considerable relevance for structural parts.
According to the prior art, the problem with forming fiber layers with φ≠0° can be avoided or can be solved in a variety of ways.
In a first variant, there is a substitution of the fiber layers φ=0° with fiber layers φ=0° while raising the number of layers for the desired firmness. Avoiding the fiber angle φ=0° however leads to disadvantageous limitations in the layered structure of the laminate. The desired firmness of the structural part thus can be realized only through a corresponding increase of the number of layers with a fiber angle of φ≠0°. This leads necessarily to an increase in weight which is especially disadvantageous in aircraft construction. The stretching of fiber layers transverse to the fiber direction can result in so-called “gaps” of an undue size between the fibers.
According to a second variant of the prior art, a deposit of single fiber layers with φ=0° directly in the profile cross section, that is, in a three-dimensional layer deposit, a subsequent forming for that layer is omitted. Depositing fiber layers with φ=0° directly in the profile cross section results in a complex production technique and expenditure requiring a large portion of manual steps for single layer deposits or, alternatively, requires a very costly mechanical process. Also, a manual lamination process is less reproducible and less cost effective. This approach runs counter to the goal of automatization and industrializing the production process.
For example, U.S. Pat. No. 7,943,076 describes a method for the production of a curved composite material structural part which includes such steps as:
providing a multitude of composite material band segments;
connecting a group of, band segments into a layer which retains this group at the layer edges within a flexible frame;
positioning the connected groups of band segments against a first curved tool surface of a mold;
contacting the connected groups of band segments with a second, generally flat tool surface of the mold;
contacting the connected groups of band segments with a third curved tool surface of the mold; and
hardening the layer.
In the example, the group of band segments, starting from a smaller radius of the curvature, can be formed with a curvature of a larger radius. The method thus exploits a wavy mold surface of an additional transitional mold, in order to transition the group of band segments into an intermediate shape. For this purpose, the transitional mold includes different mold surfaces, of which some are provided with a wave pattern formed of alternating ribs and valleys. These wave patterns are utilized to preform the prepreg into an intermediate shape whereby this intermediate shape then can be easily formed by a mold using a molding machine. The area of the prepreg that is being preformed by a first mold surface of the transitional mold is being placed against the smaller inner radius of the mold, the first curved mold surface. An adjacent area which is being preformed by a second mold surface of the transitional mold tool, and an outer cap that is being preformed by a third mold surface of the transitional mold, are formed via the mold of the molding machine.
In the foregoing variant, the forming work is considerable, since depending on the specific profile shape, successive forming of more than one edge is required. Since a specific forming tool is required, the process is thus not cost effective. In addition, such a multistep forming process can result in unwanted shifts of the fiber layers which will also have a negative impact on the reproducibility and the quality of the structural part. Due to the high degree of forming, a wave shaped structure requires a production dependent unfavorably high degree of forming wave shapes. Upon positioning connected groups of band segments against the first curved surface of a mold, in particular, groups of great thickness and a tool surface with a small curve radius, unwanted fiber compression can hardly be avoided.
It would therefore be desirable and advantageous to provide an improved production method to obviate prior art shortcomings and to provide an efficient and reproducible production method for high quality curved fiber composite profiles and to realize all types of layer structures with the aid of simple standard production methods.
The present invention resolves prior art problems of multistep processes by providing a prefabricated preform that already includes the complete fiber layer structure of the later profile and by means of the defined relevant areas can be advantageously transitioned into the profile in a one-step process.